[mle] centralize and unify address registration logic (#12864)

This commit introduces `Mle::ShouldRegisterUnicastAddrWithParent()` to
centralize the logic for determining which unicast addresses should be
registered with the parent.

Previously, the filtering logic for unicast addresses was duplicated
in `HasUnregisteredAddress()` and `AppendAddressRegistrationTlv()`. By
unifying this in a single helper method, the code ensures consistent
behavior between checking for unregistered addresses and actually
appending them to the MLE messages.

Additionally, this change:
- Marks `Mle::HasUnregisteredAddress()` as `const`.
- Updates `Mle::ShouldRegisterMulticastAddrsWithParent()` to improve
  readability and follow common coding patterns in the codebase.

src/core/thread/mle.cpp

@@ -923,32 +923,34 @@ const LeaderData &Mle::GetLeaderData(void)
     return mLeaderData;
 }
 
-bool Mle::HasUnregisteredAddress(void)
+bool Mle::HasUnregisteredAddress(void) const
 {
-    bool retval = false;
+    bool has = false;
 
     // Checks whether there are any addresses in addition to the mesh-local
-    // address that need to be registered.
+    // address that need to be registered with the parent
 
     for (const Ip6::Netif::UnicastAddress &addr : Get<ThreadNetif>().GetUnicastAddresses())
     {
-        if (!addr.GetAddress().IsLinkLocalUnicast() && !IsRoutingLocator(addr.GetAddress()) &&
-            !IsAnycastLocator(addr.GetAddress()) && addr.GetAddress() != GetMeshLocalEid())
+        if (addr.GetAddress() == GetMeshLocalEid())
         {
-            ExitNow(retval = true);
+            continue;
+        }
+
+        if (ShouldRegisterUnicastAddrWithParent(addr))
+        {
+            ExitNow(has = true);
         }
     }
 
-    if (!IsRxOnWhenIdle())
+    if (ShouldRegisterMulticastAddrsWithParent())
     {
-        // For sleepy end-device, we register any external multicast
-        // addresses.
-
-        retval = Get<ThreadNetif>().HasAnyExternalMulticastAddress();
+        // We register any external multicast addresses.
+        has = Get<ThreadNetif>().HasAnyExternalMulticastAddress();
     }
 
 exit:
-    return retval;
+    return has;
 }
 
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
@@ -1077,6 +1079,24 @@ exit:
     return;
 }
 
+bool Mle::ShouldRegisterUnicastAddrWithParent(const Ip6::Netif::UnicastAddress &aUnicastAddress) const
+{
+    bool                shouldRegister = false;
+    const Ip6::Address &address        = aUnicastAddress.GetAddress();
+
+    VerifyOrExit(!IsFullThreadDevice());
+
+    VerifyOrExit(!address.IsLoopback());
+    VerifyOrExit(!address.IsLinkLocalUnicast());
+    VerifyOrExit(!IsRoutingLocator(address));
+    VerifyOrExit(!IsAnycastLocator(address));
+
+    shouldRegister = true;
+
+exit:
+    return shouldRegister;
+}
+
 bool Mle::ShouldRegisterMulticastAddrsWithParent(void) const
 {
     // When multicast subscription changes, SED always notifies
@@ -1085,12 +1105,21 @@ bool Mle::ShouldRegisterMulticastAddrsWithParent(void) const
     // parent of 1.2 or higher version as it could depend on its
     // parent to perform Multicast Listener Report.
 
-    bool shouldRegister = !IsRxOnWhenIdle();
+    bool shouldRegister = false;
+
+    if (!IsRxOnWhenIdle())
+    {
+        ExitNow(shouldRegister = true);
+    }
 
 #if (OPENTHREAD_CONFIG_THREAD_VERSION >= OT_THREAD_VERSION_1_2)
-    shouldRegister |= !IsFullThreadDevice() && GetParent().IsThreadVersion1p2OrHigher();
+    if (!IsFullThreadDevice() && GetParent().IsThreadVersion1p2OrHigher())
+    {
+        ExitNow(shouldRegister = true);
+    }
 #endif
 
+exit:
     return shouldRegister;
 }
 
@@ -3638,9 +3667,12 @@ Error Mle::TxMessage::AppendAddressRegistrationTlv(AddressRegistrationMode aMode
 
     for (const Ip6::Netif::UnicastAddress &addr : Get<ThreadNetif>().GetUnicastAddresses())
     {
-        if (addr.GetAddress().IsLoopback() || addr.GetAddress().IsLinkLocalUnicast() ||
-            Get<Mle>().IsRoutingLocator(addr.GetAddress()) || Get<Mle>().IsAnycastLocator(addr.GetAddress()) ||
-            addr.GetAddress() == Get<Mle>().GetMeshLocalEid())
+        if (!Get<Mle>().ShouldRegisterUnicastAddrWithParent(addr))
+        {
+            continue;
+        }
+
+        if (addr.GetAddress() == Get<Mle>().GetMeshLocalEid())
         {
             continue;
         }

src/core/thread/mle.hpp

@@ -2323,11 +2323,12 @@ private:
     void       HandleChildUpdateResponseOnChild(RxInfo &aRxInfo);
     void       HandleDataResponse(RxInfo &aRxInfo);
     Error      HandleLeaderData(RxInfo &aRxInfo);
-    bool       HasUnregisteredAddress(void);
     uint32_t   GetAttachStartDelay(void) const;
     void       SendAnnounce(uint8_t aChannel, AnnounceMode aMode);
     void       SendAnnounce(uint8_t aChannel, const Ip6::Address &aDestination, AnnounceMode aMode = kNormalAnnounce);
     bool       IsNetworkDataNewer(const LeaderData &aLeaderData);
+    bool       HasUnregisteredAddress(void) const;
+    bool       ShouldRegisterUnicastAddrWithParent(const Ip6::Netif::UnicastAddress &aUnicastAddress) const;
     bool       ShouldRegisterMulticastAddrsWithParent(void) const;
     Error      ProcessMessageSecurity(Crypto::AesCcm::Mode    aMode,
                                       Message                &aMessage,